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A Middle Oxfordian to Tithonian transgressive/regressive 2nd-order sequence is recognized over most of the Lusitanian Basin of Portugal. This paper describes the nature of this sequence and its constituent 3rd-order sequences in the Arruda subbasin situated 30 km north of Lisbon. Lack of good outcrops precludes the identification of 3rd-order sequences in the transgressive part of the 2nd-order sequence, but they are easily identified in the regressive part in four different tectonic settings.

The transgressive part of the 2nd-order sequence is related to rift movements that created the subbasin. During rift initiation, carbonate depositional systems dominated. These were drowned during the rift climax phase when footwall uplift caused local erosion and karstification, and the influx of coarse siliciclastic sediments near active faults. 2nd-order maximum flooding occurred during the Late Oxfordian bimammatum zone at which time the subbasin was relatively starved of sediment and was a deep depression.

Third-order sequences deposited during the immediate postrift phase (i.e., at the beginning of the 2nd-order regression) are aggradational lowstand arkosic submarine fan deposits. As accommodation was reduced by sedimentation, localized transgressive to highstand reefal carbonates formed on the shallow proximal part of the fan. The late postrift phase was heralded by progradational sequences consisting of lowstand finegrained slope deposits capped by transgressive/highstand coral boundstones and oolites. These filled the basin virtually to sea level, so that succeeding 3rd-order sequences lacking lowstands developed in shallow, low-energy carbonate and siliciclastic fluvial facies.

Limited biostratigraphic control suggests that the 11 3rd-order sequences may be co-eval with those recognized elsewhere in Europe. If this is correct, the ages of these European sequences suggest that the rift climax event in the Arruda subbasin lasted only 1–2 my, and that subsidence rates approached 2m/ky in the center of the subbasin.

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